April 12, 1998

 

 

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EXECUTIVE SUMMARY

DC brushless fans provide a reliable and cost-effective solution for providing airflow within SET's Ping Pong Ball System (PPBS). This document is intended to provide an overview of brushless fan operation, the merits of these devices, and their limitations. Finally, consideration will be given to fan modification and resulting performance improvements, which may benefit future versions of the PPBS.

DC brushless fans and conventional DC fans are virtually identical in their outward appearance, and rely on the same physical principles in converting electrical energy into motion. They differ significantly, however, with respect to internal construction.

Figure 1 - Conventional DC Motor Commutation

The term "brushless" refers to the means of commutation employed by a fan or motor. (For the remainder of this document, motor and fan may be used interchangeably, the latter being merely a motor with blades attached to its shaft.) Commutation is a necessary feature of DC motors - in such motors, sustained rotation is impossible, unless the direction of current flowing through the motor windings is reversed every 180° of rotation. Figure 1 (above) depicts a simplified DC motor, consisting of a single-turn winding positioned between two permanent magnets of opposing polarity. As current passes through the winding, an electromagnetic field is established. Magnetic forces compel the winding to turn until its poles align with those of the permanent magnets. Commutation guarantees that the poles never actually align - every 180° , the winding current reverses direction, causing the cycle to repeat indefinitely.

In brushless fans, commutation is not achieved through mechanical means, but instead, through the use of an electronic control circuit. By using a fixed coil, and placing permanent magnets on the rotating portion of the motor, this becomes possible. Figure 2 shows a disassembled brushless fan - the Comair/Rotron Flight 80, which is currently used in the PPBS:

Figure 2 - Flight II DC Brushless Fan

Primary components in this design are labeled for clarity:

1. N-Channel JFET (Junction Field Effect Transistor)
2. Stator Coil
3. PTC (Positive Temperature Coefficient) Thermistor
4. Stator Armature
5. Hall Effect Sensor (see text below)
6. Rotor Magnet (arrow pointing at N-S pole transition)

The fan commutation circuitry consists of two N-channel JFET's, one PTC thermistor, three diodes, and one Hall Effect IC, which is mounted directly beneath the stator armature. (Duplicate components have not been labeled in Figure 2, to keep the diagram uncluttered.)

Figure 3 - Flight II DC Circuit Schematic

Fan operation is relatively simple. A Hall Effect sensor (see Figure 3, below) is the primary control component. The fan rotor rests in the stator armature such that a circular permanent magnet, mounted inside the rotor, is in proximity with the Hall Effect sensor. When the N-S pole transition (see Figure 2) of the rotor magnet passes the Hall Effect sensor, two 50% duty cycle pulses are generated, 180° out of phase.

During normal operation, these pulses are issued repeatedly, forming two complementary pulse trains. These pulses are used to drive the two JFET's - each of which switches drive current to a separate pair of stator coils. Because the pulse trains are 180° out of phase, when one JFET is on, the other is off. Thus at any given instant, one pair of coils is energized. Alternate firing of these coil pairs in succession results in steady rotation of the fan rotor. In case of high temperature due to fault or fan stall, the PTC thermistor's resistance increases, thereby throttling coil current until fan temperature returns to a reasonable level. Two reverse-biased diodes protect the JFET's from inductive back-emf during switching, and a third protects the control circuitry, should the fan be connected with reversed polarity.

Brushless motors provide a number of advantages, particularly the following:

Brushless motors, as a consequence of their low inertia, are more prone to stalling than conventional DC motors. Because their electronic control circuitry is not tolerant of high temperatures, thermistors are typically included in brushless designs as a means of safeguarding against these conditions.

Two simple modifications can enhance the Flight 80:

Reversible operation may be achieved by providing for some means of reversing the "firing order" of the JFET's. The same thing, however, can be accomplished by swapping stator coil pairs. The following diagram shows how this can be done mechanically using a DPDT switch. If the circuit in Figure 3 is broken at the points labeled "A" and "B", and this circuit is inserted, toggling the switch will cause the fan to reverse directions. This can be controlled electrically by replacing the switch with a DPDT relay. Finally, the equivalent of a DPDT switch can be implemented using MOSFET's, to allow switching with low power digital signals.

Figure 4 - Simple Reversibility Modification

DC brushless fans are being marketed primarily for the purpose of cooling consumer electronics equipment, typically computers. Because fan failure can lead to catastrophic hardware failures, speed monitoring is becoming a popular means of detecting fan failure. In response to this demand, current fan manufacturers often provide a tachometer output - an additional lead which carries digital pulses, whose frequency is proportional to fan speed.

Adding this functionality to the Flight 80 is simple. By attaching a wire to lead 2 or 3 (see Figure 5) on the Hall Effect sensor, pulses like those described above may be obtained. This signal is very weak, so circuits or equipment accepting these pulses must have a high input impedance (10MW or greater), so that fan operation is not affected. Pulses are generated at a rate of two pulses per revolution.

The manufacturer of the Hall Effect sensor used in the PPBS fan remains uncertain, but the following pinouts have been determined by inspection:

Figure 5 - Flight 80 Hall Effect sensor

The fan is a key component in the Ping Pong Ball System - its performance is a critical factor in overall system performance, so understanding its operation will greatly facilitate efforts to improve the existing system. Simple modifications such as the two described above not only allow the fan's performance to be modified, but they also permit its internal control system to provide input to the PPBS control system. Sensor input derived from the fan control circuitry may promise the introduction of improved precision into future implementations of the PPBS control algorithm.

Comair Rotron: Engineering Notes http://www.comairrotron.com/Engineering.htm

Electric Motors Project http://www.meceng.uct.ac.za/~mec104w/projects/electricmotors/electricmotors.html

Mechatronics, Inc.  http://www.thomasregister.com/olc/enprotech-mech/brushles.htm